Mesh: fixes #7018: 3mf created with FreeCAD cannot be loaded with Pru…

[wwmayer]

…saSlicer

Thank you for creating a pull request to contribute to FreeCAD! To ease integration, we ask you to conform to the following items. Pull requests which don't satisfy all the items below might be rejected. If you are in doubt with any of the items below, don't hesitate to ask for help in the FreeCAD forum!

• Your pull request is confined strictly to a single module. That is, all the files changed by your pull request are either in App, Base, Gui or one of the Mod subfolders. If you need to make changes in several locations, make several pull requests and wait for the first one to be merged before submitting the next ones
• In case your pull request does more than just fixing small bugs, make sure you discussed your ideas with other developers on the FreeCAD forum
• Your branch is rebased on latest master git pull --rebase upstream master
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• All commit messages are well-written ex: Fixes typo in Draft Move command text
• Your pull request is well written and has a good description, and its title starts with the module name, ex: Draft: Fixed typos
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And please remember to update the Wiki with the features added or changed once this PR is merged.
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